The present invention relates in general to a pipe bending machine, and in particular to bending of pipes. To be more precise, the invention relates to pipe bending dies and the procedure for the use of the bending die according to the invention.
To bend a pipe through a bend angle .beta., a bending die mounted on a bending machine is used. Such bending dies are in the shape of cylinders whose basic surface is a circle and whose lateral surface comprises a groove whose cross section is a semi-circle with a diameter equal to the external diameter of the pipe to be bent. The pipe is applied to the bending die at the groove in the lateral surface and the pipe is turned through an angle greater than .beta., equal to (.beta.+.alpha.) since it is known that after removal of the bending force it undergoes an elastic deformation which tends to reduce the bend. In the rest of this document, the desired permanent bend will be denoted .beta. and the extra bending i.e. the extra angle through which the pipe must be turned around the bending die to obtain the permanent bend .beta. will be denoted .alpha.. For all useful purposes, it shall be remembered that the bend of a pipe is the angle formed by the neutral axes of two straight consecutive parts of a pipe, the neutral axis itself being the geometric locus of the centres of the cross sections of the pipe. The bending radius is the radius of the circumferential arc of the neutral axis contained between two consecutive straight parts. The curve described by the neutral axis between two straight parts may not be a circumference; in this case the average bending radius R will be defined by the relationship S/.beta. in which S is the length of the curve contained between straight parts and the bending. When a pipe is bent on a circular die of radius R, it is wound as explained above through an angle greater than angle .beta., the winding angle then being .beta.+.alpha.. In this was the bending radius after elastic deformation is greater it becomes: ##EQU1## The result of this is that if it is desired to bend a pipe accurately through an angle .beta., with the bending radius being R, a bending die smaller than R must be used. The extra bend .beta. is itself an increasing function of angle .beta. in such a way that the greater the desired angle of bend, the smaller the radius of the bending die to be used must be in order to keep the same bending radius. This phenomenon being known, until now sets of bending dies have been used for each bending radius and, for example one can, in order to obtain a given radius R for bends between 0.degree. and 30.degree., use a circular bending die of R1 so that R1&lt;R, then in order to bend through between 30.degree. and 60.degree. a bending die of R2, R2&lt;R1 and so on until the complete range of desired bends is obtained. The radii R1, R2 of bending dies that are used for the various sub-ranges are determined by experimentation on representative samples of the pipes that are to be bent.
The prior art, as has just been described, is illustrated by FIGS. 1 and 2.
On FIG. 1 can be seen a circular bending die 1 on which by means of a clamping jaw 2 pipe 3 is applied. The latter is wound by means of clamping jaw 2 through an angle (.beta.+a) 4 equal to angle 5 which between them make the segments of neutral axes 6 situated on either side of the bend.
FIG. 1 illustrates the first phase in pipe forming, the second phase then consists of moving the pipe forward by length .DELTA.L possibly turning it on itself, in order to perform parbuckling, then performing the following bend with possible changing of the bending die, if the desired bending angle for the following bend requires a bending die with a different radius.
FIG. 2 illustrates the new position of the pipe at the end of phase 2 and in this can basically be seen pipe 3 and its neutral axis 6 comprising two segments, on either side of the bend. Due to the fact of elastic deformation, these two segments between them form an angle 7 that is in theory equal to .beta. if the extra bend has been correctly chosen.
This prior art has two disadvantages: on the one hand it means a change in the bending die every time there is a change from one bending range to another, on the other hand it allows an error on the bending radius and/or the bending to remain due to the fact that the changes in the bending die radius (R1, R2 etc.) are discontinuous ones. This error can be a large one and may be unacceptable in the case of pipes intended to be housed in large number in cramped locations such as the hulls of submarines. Part of the error originates in the bending tool, the circular bending die, whose radius only changes in stages, and part originates in not knowing the extra bends a that need to be applied.